(1) Field of the Invention
The invention is related to a mounting arrangement for mounting at least a gear box of a rotorcraft to a fuselage of a rotorcraft, said mounting arrangement comprising the features of claim 1. The invention is further related to a rotorcraft comprising such a mounting arrangement, said rotorcraft comprising the features of claim 16.
(2) Description of Related Art
Lift and other forces that are required in order to allow a controlled flight of a rotorcraft are usually at least essentially generated through one or more rotors of the rotorcraft in a more or less horizontal plane. Conventional design methodology utilizes a rotating or non-rotating mast, to which a given rotor that is powered by an associated gear box of the rotorcraft is mounted and which is usually connected to a gear box independent support structure. The latter is sometimes also referred to as a stand pipe support structure or a lift housing, which is connected to the rotorcraft's airframe, i.e. the rotorcraft's fuselage. Alternatively, the rotating or non-rotating mast can be connected to a mast mounting integrated into the associated gear box, which in turn is connected to the airframe of the rotorcraft. Furthermore, some designs may feature load paths, wherein forces in one or more load directions are solely carried by the gear box independent support structure, while all other forces are routed through the gear box.
It should be noted that in the context of the present invention all arrangements where all bearings that transmit non-torque rotor forces and moments from rotating elements to static elements are mounted above gears of a given gear box of a rotorcraft are considered as having a gear box independent support structure, even if the latter is integrated into a part of a respective gear box housing. A respectively selected design is usually merely dependent on an underlying rotorcraft layout and other applicable factors. Such factors e. g. comprise a selected height above the airframe of the rotorcraft that the gear box and gear box independent support structure impose on the rotor and it is generally considered that any rotor height above a given height that is necessary to accommodate movement and deformation of the rotor in relation to the airframe adds unnecessary aerodynamic drag.
In operation of a rotor of a rotorcraft, large magnitude forces and moments are generated and acting on the rotor that is powered by an associated gear box. It is commonly known that the highest magnitude forces generated by the rotor are those perpendicular to an associated rotor plane, i.e. lift forces. The moments of highest magnitude generated by the rotor are oriented around an axis running on the associated rotor plane. Such large magnitude forces and moments must be transferred to the airframe of the rotorcraft in order to achieve a controlled flight of the rotorcraft while guaranteeing a safe, reliable and durable operation. Furthermore, conversion of rotation speed and torque in the associated gear box also generates loads that must be transferred to the airframe of the rotorcraft. Moreover, inertial loads generated by the associated gear box during landing maneuvers or crash landings of the rotorcraft must also be carried by the airframe.
Frequently, the gear box and the rotor of the rotorcraft are mounted to its airframe by means of a single, i.e. collective mounting arrangement. Furthermore, a variety of anti-vibration functions can be achieved in the form of vibration isolation means that are arranged between the gear box/mounting arrangement and the airframe in order to diminish lifetime reducing vibration loading on rotorcraft equipment and in order to meet ever increasing customer requirements and state regulations concerning safe working environments. In other words, the gear box/rotor is mostly mounted to the airframe by an associated mounting arrangement that is suitable for accommodating anti-vibration means mounted between the gear box/rotor and the airframe.
It should be noted that, as with all elements of a rotorcraft, a lightweight design of such a mounting arrangement is of utmost importance. It is obvious that a favorable selection of geometry and material for all components of the mounting arrangement is therefore of essence. Furthermore, it is generally considered that a compact design is of low weight due to minimization of load carrying distances. Furthermore, a cost efficient solution is generally preferred in order to achieve commercial viability.
More specifically, conventional gear box designs generally use a cast metal or machined metal housing for mounting of the gear box and for transferring rotor loads that are occurring in operation. The metals used are mainly aluminum and magnesium. Such cast metals are, however, of comparatively low strength and have comparatively high defect ratios that must be taken into consideration, and are therefore comparatively heavy compared to machined metals. Machining, on the other hand, places higher restrictions on a possible gear box housing geometry, while allowing slightly higher stress levels. Both methods have a limit on how thin geometries can be manufactured.
Furthermore, structures of gear box housings are usually adapted for separating their interior from the exterior. Therefore, such structures are oversized regarding stress levels as a consequence of an underlying minimum thickness restriction due to cost or manufacturing technology. Use of heavier materials with a higher strength to weight ratio is, therefore, not beneficial as further reduction of the wall thickness of the wall of the gear box housings is not possible and, consequently, a large weight penalty is accumulated in low stress regions. A combination of a lightweight material cast or machined housing with a high strength mounting means is, therefore, preferable. On the other hand any existing structure oversized for stress should be used as far as possible instead of adding additional elements. As composite materials are known for their high strength to weight ratio and as they exhibit comparatively good fatigue characteristics, it is common engineering practice to utilize these materials in rotorcraft design.
Another issue that should be considered in the design of a suitable mounting arrangement is that aside from connecting driving elements of a given rotorcraft with its lift generating rotor, the gear box is also used to drive various other devices and systems of the rotorcraft. The weight and cost conscious integration of these devices and systems is, thus, relevant for design of a suitable mounting arrangement.
It should be noted that the term “mounting arrangement” refers in the context of the present invention to all non-rotating elements that are used to transfer rotor loads and potentially gear box loads to the rotorcraft's airframe. Exemplary mounting arrangements are described hereinafter.
The document EP 0 508 938 A1 describes with respect to FIG. 1B an exemplary mounting arrangement that comprises a stand pipe support structure for a main rotor assembly of a rotorcraft with a plurality of attachment feet. This stand pipe support structure supports a non-rotating rotor mast and a gear box of the main rotor assembly of the rotorcraft, which are integrated with an attachment collar of the stand pipe support structure. In this type of stand pipe support structure, a gear box housing of the main rotor assembly is at least partly defined by a body of the stand pipe support structure, including the attachment feet. The non-rotating rotor mast is integrated with the attachment collar. The stand pipe support structure is secured to an upper deck region of the rotorcraft by means of bolts passing through the attachment feet. Dynamic and static loads of the main rotor assembly are transmitted to a single load transfer level of the rotorcraft's airframe, i.e. the upper deck region, via the attachment feet.
This stand pipe support structure can be manufactured comparatively easily and with low costs of fabrication as an integral unit, and can easily be mounted to an upper deck region of a rotorcraft. Furthermore, due to a relatively uncluttered configuration of this stand pipe support structure, hydraulic lines, subsystem wiring and other interfacing elements that are typically routed over the upper deck region may be readily run over/adjacent to an exterior surface of the stand pipe support structure.
However, this stand pipe support structure is disadvantageous with respect to integration of the gear box within the stand pipe support structure, as the gear box housing acts as a structural member through which dynamic and static loads of the main rotor assembly are intermediately transmitted. Moreover, the weight of the stand pipe support structure is comparatively large because of a required high structural strength of the stand pipe support structure and a usually low strength of the used materials. Furthermore, the direct attachment of the gear box housing to the upper deck region does not provide significant room for provision of a suitable anti-vibration device.
The document EP 0 508 938 A1 also describes with respect to FIG. 1C a further exemplary mounting arrangement that comprises a strut support structure for a main rotor assembly of a rotorcraft. This strut support structure is embodied as a high profile configuration with an integration member and a plurality of struts, such as cylindrical rods or machined legs extending from the integration member and terminating in attachment feet. In this strut support structure, a non-rotating rotor mast of the main rotor assembly is attached to the integration member in a manner that is similar to the one described above with respect to the stand pipe support structure. However, the gear box, i.e. its gear box housing, is attached in suspended combination to an underside of the integration member and, consequently, is not part of a load path for transfer of dynamic and static loads of the main rotor assembly. Instead, the attachment feet are utilized to secure the strut support structure to the rotorcraft's airframe and to transfer dynamic and static loads of the main rotor assembly to respective hard points on the airframe.
The main advantage of such a mounting arrangement concerning load path and material optimization is the possibility of utilizing a high strength to weight ratio material in the strut support structure and the plurality of struts due to not being bound by respective requirements of gear box design. However, a required minimum wall thickness of the gear box housing combined with it not being used for loads transfer results in comparatively low stress levels and, consequently, wasted material and weight. Furthermore, routing of hydraulic lines, electrical subsystem wiring and other interface components along a respective upper deck region of the rotorcraft is complicated because of the strut network that must be accommodated thereon. Finally, an anti-vibration device can easily be integrated into the struts of the strut support structure, but due to a comparatively large number of struts, this would be costly and weight intensive.
The document US 2007/0034736 A1 describes another exemplary mounting arrangement that comprises a strut support structure for a main rotor assembly of a rotorcraft. This strut support structure reduces the underlying number of struts required for load transfer in comparison to the above described strut support structure by utilizing a support arrangement below the gear box for loads running approximately parallel to a rotor plane of the main rotor assembly. More specifically, rotor and gear box of the main rotor assembly are held along the rotor axis by four angled struts that are connected to a lift housing atop the gear box. The lift carrying bearing in this design is situated at the top of the lift housing where the struts are attached. Due to the offset of respective strut attachments to the rotor axis, a part of the moments generated in the main rotor assembly's rotor mast by the rotor are absorbed by the struts as well, but this results in large forces in the struts. However, the strut support structure is not suited for transferring torque loads parallel to the rotor plane, as the struts are situated more or less radial to the rotor axis. Such torque loads and remaining moments are, therefore, countered by the support arrangement below the gear box, which is designed in order to allow vertical freedom of movement, but no rotation around the rotor axis. Movement perpendicular to the rotor axis may be allowed for accommodating an anti-vibration function.
An example of a device that is suitable for allowing such a perpendicular movement is described in the document U.S. Pat. No. 3,920,202. Another option is the use of a membrane.
The mounting arrangement according to document US 2007/0034736 A1 is simple and comprises only a few constituent components with comparatively small complexity. Furthermore, its struts can easily be adapted to end on strong structural elements of the rotorcraft's airframe, such as intersections of longerons and frames. Moreover, an anti-vibration device can easily be mounted below the gear box, if desired. Otherwise, a simple and cheap strut arrangement can be achieved. Alternatively, an anti-vibration device can be fitted into the angled struts as well.
However, due to the effects of the strut geometry, flat struts would result in high strut loads and, therefore, require heavy and large struts. This can be avoided by utilizing a high height and low diameter gear box or by adding an additional support structure above the gear box, which would, nevertheless, lead to a bulky and cumbersome design. Additionally, this height requirement may conflict with a need for a low mounted rotor that is required for optimized aerodynamics.
Still another mounting arrangement is known from the Airbus Helicopters rotorcraft H135. This mounting arrangement comprises a rotating rotor mast and a gear box housing that consists of an upper and a lower housing. Suitable bearings for bearing the rotor mast in a rotatable manner are integrated into the upper and lower housings. The upper housing furthermore comprises, at two opposing sides, attachment structures that are integrally formed with the upper housing for attachment of so-called z-struts, which are adapted for transfer of lift forces. So-called x-struts, which are adapted for transfer of torque and in-plane rotor forces, are connected to bottom parts of the triangular attachment structures and a so-called y-strut, which is adapted for transfer of in-plane rotor forces, is attached to a separate bracket on the lower housing. The gearbox housing comprises dimensions that are chosen such that all required gear box internals can be installed therein. Consequently, these dimensions also define a minimum distance between the two triangular attachment structures, which can be larger than required for the struts or a possible anti-vibration device. At the same time, a possible choice of gear box materials is hindered by the minimum wall thickness issue of the gear box housing.
Advantageously, this mounting arrangement is comparatively flat and utilizes the required minimum wall thickness of the gear box housing to its advantage, as it carries rotor loads. However, if the distance between the two triangular attachment structures is larger than required for the struts, this results in unnecessary long load paths and induces a weight penalty, i.e. an unnecessary large gear box weight.
Still another mounting arrangement is known from the Airbus Helicopters rotorcraft H145. This mounting arrangement also comprises a rotating rotor mast and a gear box housing that consists of an upper and a lower housing, wherein suitable bearings for mounting the rotor mast in a rotatable manner are integrated into the upper and lower housings. However, in contrast to the above described mounting arrangement that is known from the Airbus Helicopters rotorcraft H135, four V-shaped brackets are attached to the upper and lower housings for transferring the lift forces. The V-shaped brackets are attached to the rotorcraft's airframe with struts and connected to the gear box housing by means of screws. This screw connection is, however, not directly adjacent to the bearings for the rotor mast. Furthermore, additional struts are provided at the bottom of the gear box housing for transferring the torque and drag loads.
Advantageously, this mounting arrangement allows parts of the strut support structure to be made of a material other than that used for the gear box housing thereby utilizing optimized material selection. Furthermore, due to the differential nature of the design of the mounting arrangement, an underlying placement of the struts is less constraint by the gear box internals and the gear box housing. However, due to geometrical constraints, the V-shaped brackets are not mounted at a respective source of the loads that are introduced in operation of the rotorcraft into the gear box housing, namely at the upper and lower mast bearings. The gear box housing, therefore, has to carry all loads between the V-shaped brackets and the bearings for the rotor mast for a distance. Furthermore, the attachment of the V-shaped brackets with close set screws lead to a stress concentration in the gear box at corresponding positions of the screw connections.